Bypass valves may be provided in drill strings to provide a flow path between the drill string bore and the annulus without the requirement for fluid to pass through elements of the bottom hole assembly (BHA). This may be useful for a number of reasons. When it is considered appropriate or necessary to deliver lost circulation material (LCM) to the annulus, it is preferred that measurement-while-drilling (MWD) tools and the jetting nozzles in the drill bit are isolated from the LCM, which might otherwise cause damage or blockage. Thus, a bypass valve may be provided above the MWD tool. Furthermore, for hole cleaning it may be desirable to achieve a higher circulation rate of fluid in the annulus above the valve, and this is more readily obtained if the circulating fluid can bypass the drill string and MWD below the bypass valve which would otherwise consume pressure and thus hydraulic power.